trading stamp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A stamp given by a retailer to a buyer for a purchase of a specified amount and intended to be redeemed in quantity for merchandise.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a small paper coupon, given by a retailer to a customer; they may be saved up and redeemed for goods

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a token resembling a stamp given by a retailer to a buyer; the token is redeemable for articles on a special list